1. Field of the Invention
Embodiments of the subject matter disclosed herein generally relate to methods and systems for pumping/compressing a multiphase fluid.
2. Description of the Prior Art
In recent years, with the increase in price of fossil fuels, the interest in developing new production fields has increased. Drilling onshore or offshore poses various problems. One such problem is that a petroleum fluid that comes out of a well comprises at least first and second components. The first component may be a gas and the second component may be a liquid. In addition, the gas component may not dissolve and/or mix into the liquid component. Thus, the petroleum fluid is a multiphase fluid.
Pumps and compressors are used in the industry for extracting the petroleum fluid from the well or for transporting it along a pipe. A pump is typically used for transporting a liquid while a compressor is used for transporting a gas. For these reasons, the pumps are designed to be efficient for liquids while the compressors are designed to be efficient for gases. Because of the different compositions of the gas and liquid and different laws of physics applying to these fluids, a pump is not efficient when a gas is present in the mixture and a compressor is not efficient when a liquid is present in the mixture.
Thus, for handling a multiphase fluid (e.g., a fluid that comprises at least a gas and a liquid component) it is customary to use various pumps connected in series. In this regard, U.S. Pat. No. 5,961,282 (the entire disclosure of which is incorporated by reference herein) discloses a system that comprises an axial-flow pump connected via a connecting part to a centrifugal pump.
An axial-flow pump, as the name suggests, imparts energy or pressure to a liquid that travels along an axial direction of the pump. For illustration, FIG. 1 shows an axial pump 10 having a casing 12 in which a statoric part 14 is configured to be provided about a shaft 16 and to deflect an incoming liquid. An impeller 18 is configured to rotate with shaft 16 and to direct the accelerated liquid. If shaft 16 is considered to extend along axis Z, then the liquid exiting the impeller 18 has substantially a speed v along axis Z. This property of the liquid exiting the impeller to move substantially along axis Z determines a pump to be axial-flow pump, i.e., the output liquid flows along the axis of the pump.
On the other end of the spectrum, a centrifugal pump makes the liquid exiting the impeller flow substantially radially from the axis of the pump, as shown in FIG. 2. FIG. 2 shows a centrifugal pump 20 in which a liquid is output with a speed v along axis X, radially from the axis of the pump that lies on Z. The liquid is shown entering along arrow A at an inlet 22.
Turning to U.S. Pat. No. 5,961,282, this reference discloses using a system 30 (see FIG. 3 which corresponds to FIG. 2B of U.S. Pat. No. 5,961,282) having an axial pump 32 and a centrifugal pump 34. A fluid enters inlet 36 and is acted upon by impeller provided after a statoric part 38. After passing the axial pump 32, as the fluid has a speed substantially parallel to a shaft 40, an adjuster 42, fixed to a casing 44, is used to deviate the incoming fluid to enter passage 46 (input) of the centrifugal pump 34 at a speed substantially perpendicular to the shaft 40. Blade 48 of the centrifugal pump 34 further imparts energy or pressure to the liquid and also changes the flow direction along a direction X perpendicular to the axis of the pump.
With the methods of the above noted reference and other references, a petroleum effluent is transported from, for example, the bottom of the well to the surface by using a pump system that comprises a set of front stages of helicoaxial type, complemented with a set of back stages of the radial type (centrifugal stages). The two sets of stages may be stacked on the same axis.
Centrifugal stages are able to efficiently pump single-phase liquids only in the absence of a gas phase. As soon as the Gas-Volume-Fraction (GVF), which measures the ratio of gas to liquid phase volume rates, exceeds a few percent, conventional centrifugal stage performance deteriorates and prevents safe operation of the pump. To avoid this problem, the GVF is reduced by means of a set of axial stages, e.g., helicoaxial for the front stages, and radial stages for the last stages. The front set of helicoaxial stages are tolerant to high GVF, and they are able to gradually reduce the GVF through moderate pressure increase prior to reaching the last set of radial stages that are operated with a lower GVF. The first set of helicoaxial stages are capable of handling large GVF, but at the expense of a reduction in the pressure increase per stage. This solution requires an increase in the overall number of stages to reach the desired discharge pressure which increases weight, shaft length and cost.
Accordingly, it would be desirable to provide systems and methods that are better than the systems discussed above.